The present invention concerns a fixed spike on a sleevemounted spike, fitted on a vehicle tyre.
Increasing traffice load and spike tyres in combination have proven to be a remarkable road attrition factor. In some countries this has even led to prohibition of spike tyres, or at least to considerable restrictions.
In Nordic conditions, the beneficial effect of anti-slip means on the safety and flexibility of traffice has on the other hand been irrefutably demonstrated, and this effect should not be sacrificed: instead, the associated drawbcks should be eliminated. Good results will be achieved by further developing both the road superstructures and the anti-slip tyres.
As a pneumatic automobile tyre rolls on an even surface, it is considerably flattened radially, owing to its flexibility, whereby in the contact region longitudinal as well as transversal forces are generated owing to changes of the rolling radius.
The longitudinal forces acting on the spike when the tyre is rolling are due to bending of the body structure, to longitudinal slipping and to the stress wave in the rubber.
When a spike approaches the point of contact with the road, the tyre body undergoes bending such that the radius of the bent part is significally smaller than that of equivalent parts of the load-free tyre. This deflects the spike, which has been mounted at right angles against the surface, to assume a vertical position before contact with the road. Owing to the protrusion of the spike point, however, the spike is not turned into sufficiently upright position: it meets the road surface in an oblique position. At this stage, he forces due to slipping tendency also begin to exert their influence. As the rotating tyre surface and the protruding body (the spike) attached thereto meet the road surface, a dynamically caused impact against the road surface is produced. The attrition of the road can be significally reduced if said impact can be eliminated or reduced.
The impact has been found to be due to the kinetic energy of the spike, in other words, due to the mass of the spike and its velocity in the direction against the road surface. The effective mass is influenced by the spike mass itself and, in addition, by a friction force, its magnitude depending, in its turn, on the spike design in that the friction of a spike body mounted in a sleeve results from sliding between the sleeve and the metallic body, while the friction force of a fixed spike is active between the spike body and the rubber. The mass of a fixed spike is influenced, owing to its shaping, partly also by the mass of the surrounding rubber.
Prior designs have not eliminated the abrasion effect from the dynamic impact in any other way than by reducing the mass of the spike, and it is believed that the practical minimum limit has already been reached in this respect. As taught by the new invention, certain design features are suggested for the spike and the tyre with the express aim to reduce the dynamic initial contact peak and the ultimate scratch.
Traditionally, the shape of the spike has been symmetric and it has been mounted in a hole, perpendiculalr against the wear surface of the tyre, whereby it meets the road in a slightly oblique position as the forces mentioned affect its position. The oblique contact of the spike with the road, as well as its being pushed deeper into the tyre in oblique position during the initial part of road contact, cause damage to the rubber and to the spike which impairs the friction properties of the spike, detracts from the durability of the spike and increases its road attrition properties.